Generating stimulated emission normally requires that a population inversion be established between two energy levels (also referred to as states in the art) of an atom or molecule. Populating the upper laser level cannot be accomplished directly from the lower laser level, but rather requires the presence of at least one highly excited level through the pumping process and pump power must flow. Three level laser systems employ one highly excited level and lasing can occur between two excited levels or, alternatively, the lasing transition can terminate at ground. FIG. 1A illustrates the principles of a three level system in which the ground level of the lasing species is the terminus of the lasing transition.
From a conceptual perspective, three level systems are the simplest in existence, and provide a convenient example to illustrated laser pumping theory. Virtually all three level lasers demonstrated to date (and all continuous wave [CW] lasers) have the generalized energy structure illustrated in FIG. 1A. An electrical or optical pump source excites and transfers atoms (or molecules) from Level 1 (sometimes referred to as a lower laser level) to Level 2 (sometimes referred to as a highly excited level). Following a fast, radiationless relaxation of the Level 2 population to Level 3 (sometimes referred to as an upper or upper metastable laser level) by any of a variety of processes (collisions, multiphonon interactions, fluorescence, etc.), lasing occurs on the 3→1 transition via a relatively slow transition. Although this simple system has proven to be highly successful and provided practically useful devices for many applications, it often suffers from several drawbacks. The drawbacks include low quantum efficiency and inefficient relaxation of Level 2.
The quantum efficiency for the laser of FIG. 1A is limited by ΔE, the energy separation between Levels 2 and 3. A significant difficulty is that the energy ΔE is dissipated as heat. That is, the fraction:
                                          Δ            ⁢                                                  ⁢            E                                E            2                          =                ⁢                                            E              2                        -                          E              3                                            E            2                                                  =                ⁢                  1          -                                    E              3                        /                          E              2                                          
represents the percentage of the energy of every pump photon (assuming that the laser is optically pumped) that is not recovered in the laser output. For a typical three level laser, this percentage is at least 5-10%. In many applications, this energy loss not an important concern, e.g., lasers that are pulsed because the laser is “off” much longer than it is “on.”
High power lasers that are continuous or have a duty cycle (the percentage of time that the laser is operational) greater than 5-10%, present different problems. In continuous and high duty cycle lasers, the rejection of heat resulting from ΔE in FIG. 1A can be (and generally is) a serious problem. At a minimum, heating of the laser medium will distort the output laser beam. In cases where the laser medium is a crystal, catastrophic damage of the crystal can result. Inefficient relaxation is also another problem. The excitation transfer step (radiationless transition from Level 2 to Level 3) is difficult to implement as hydrocarbon, which is used to relax the population from Level 2 to Level 3, causes pyrolyzing problems that produces “soot” in the laser.
Efficiently relaxing the population of Level 2 into Level 3 can also present serious engineering issues. For example, three level lasers in the akali atoms Cs and Rb have been developed that require the use of a hydrocarbon molecule such as ethane (C6H6) to relax the population of Level 2 into Level 3. However, because the laser medium must be heated so as to obtain a suitable pressure of the alkali metal vapor, the hydrocarbon will slowly pyrolyze (decompose) as the temperature system is raised.
DPAL (diode-pumped alkali lasers) are a newer class of laser that pump atomic alkali vapors with diode arrays. See, e.g., Krupke et al, “Multimode-diode-pumped gas (alkali-vapor) laser,” Optics Letters, Vol. 31, Issue 3, pp. 353-355 (2006). In that example, a volume-Bragg-grating stabilized pump diode array pumped Rb vapor. The laser operated on the 795 nm resonance D1 (lasing) transition. Prior work by Krupke et al. used a titanium sapphire laser as a pump to create population inversions and laser operation on the 795 nm resonance D1 (lasing) transition of Rb. These laser systems also pump the 1→2 transition.
Increasing the DPAL lasers to the kW power level and beyond would be difficult. Various engineering barriers exist. For example, the Rb D2 Linewidth (broadened by ˜1 atm He or ethane)≈10 GHz. Linewidth narrowing is needed with volume Bragg gratings. Electronic stabilization of the diode array wavelength is also required.
The laser transition defines different types of laser system. In conventional three level systems the laser transition is to ground Level 1 (the lower laser level in a three level system). There are also four level laser systems. An example medium that provides four level operation is Nd:YAG. In a conventional four level laser, the laser transition is to a lower laser level slightly above the ground level. A natural depopulation then occurs to the ground level, and this is another fast radiationless transition. In the conventional three and four level systems, the population inversion is achieved in the same manner. Energy pumps population from 1→2, as shown in FIG. 1A with respect to a three level laser.
Some lasers are optically pumped, e.g., the DPAL lasers that are pumped with diode arrays. The 1→2 pumping transition of FIG. 1A presents another potential obstacle to obtaining efficient operation of the laser system when the system is optically pumped. In an optically pumped system, the pumping transition from 1→2 suffers from poor absorption. Pump energy is wasted, rendering the laser system less efficient.
Verdeyen, Eden, Carroll, Readle and Wagner U.S. Pat. No. 7,804,877 is directed toward Atomic Lasers with Exciplex Assisted Absorption. In the '877 Patent, the optical cavity includes a van der Waals complex of an alkali vapor joined with a polarizable rare gas. The pair is referred to as an exciplex. An example pair is the CsAr pair illustrated in FIG. 6 of the '877 Patent. The generalized pair is illustrated in FIG. 5. The primary examples are alkali-rare gas pairs, though mercury and other polarizable molecules such as ethane and methane are identified as possible substitutes for the polarizable rare gas molecule. The alkali-rare gas atomic pairs are photo pumped in the band known as the blue satellite for the D2 transition of the alkali atom. As a result, the atomic pair is excited (promoted in energy) to the repulsive B state of the alkali-rare gas diatomic molecule. The rapid dissociation of the molecules in this B state results in populating an excited state of the alkali atom that serves as the upper state for the D2 transition of the alkali atom. A similar process can be used to populate higher-lying excited states of the alkali atom. Lasing is obtained on at least two transitions of an alkali atom without necessity of collision relaxation of one level to the other. Rapid dissociation of the alkali-rare gas molecule is used to populate the upper laser level. The lasers and pumping method of the '877 Patent use a single pump that is away from the atomic resonance on the satellite band. In a transversely pumped example of FIG. 11, diodes are on opposite sides of the medium, but this is for uniform pumping and is in the satellite band.
Efficient operation (or any lasing at all) within the methods of the '877 Patent requires that the separation in energy between the selected satellite pump band and laser photons must be greater than, or comparable to, kT (thermal energy). An example consistent with the methods and systems of the '877 Patent is shown in FIG. 1B which is an absorption spectrum recorded for a mixture of rubidium (Rb) vapor and Xe gas. The optical transmission through a column of this vapor/gas mixture is given as a function of wavelength in the 755-785 nm spectral region. Absorption increases downward in this graph, and the positions of the D2 line of Rb (wavelength of approximately 780 nm) and the blue satellite of the D2 line of Rb in Xe are also indicated. The energy separation between the peak of the blue satellite and the D2 line position is 337 wavenumbers (1/cm), or approximately 42 meV, which is 7% greater than kT for the temperature (473 K=200 degrees Centigrade) at which the data of FIG. 1B were obtained. Consequently, because the separation between the blue satellite of the D2 line and the D2 line itself is more than kT in energy, optically pumping the blue satellite results in lasing on the D2 line. If the system is pumped closer to the D2 line (for example between 765 and 780 nm in FIG. 1B), lasing will not occur.
Most research efforts are directed toward improving the performance of traditional three and four level laser systems by enhancing the efficiency of the pumping mechanism. It is well known that a population inversion is necessary to realize a laser, but that direct pumping of the upper laser level from the lower laser level (which can be ground level or an elevated level) will not yield a population inversion between the two levels. Instead, the two levels can be equalized in theory that assumes use of an infinite pump source. Thus, theory instructs that a two level system cannot be inverted, i.e., the population of the more energetic of the two levels cannot exceed that for the lower of the two levels. With a strong pump source, the populations of the two levels can, at best, be equalized. This fails to produce the population inversion needed for lasing.